Process for treating electrodes



Patented Sept. 19, 1933 UNITED STATES rnocsss Foa TREATING nmc'monnsRalph M. Hunter and Louis E. Ward, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich, acorporation of Michigan No Drawing. Application February 2, 1931 SerialNo. 513,049

8Claiml.

This invention provides an improved process for treating carbon orgraphite electrodes, thereby making the same more resistant todeterioration and lengthening the life of/or otherwise materiallybenefiting the electrodes thus treated when utilized in the usual way,such as in the electrolysis of an aqueous solution of sodium chloride.

In electrolytic cells of the bi-polar type for the manufacture ofcaustic soda and chlorine, employing carbon or graphite electrodes whichhave a characteristically porous structure, it has been customary toimpregnate the electrode with an oil or wax to seal the pores so as toprevent the diffusion therethrough of gaseous prodnets of electrolysis.Hydrocarbon oils or waxes have been used for the purpose, but dryingoils, particularly China-wood or tung oil, have been found to give thebest results. Certain difiiculties have been experienced with the use ofsuch impregnated electrodes, however, arising chiefly from the gradualchlorination of the oil or wax in service. The chlorination appears tobe accompanied by some expansion in volume. This action causes sweatingor exudation of oil from the pores, and such oil, becoming detached fromthe electrodes, mixes with the electrolyte and is eventually carried tothe diaphragm, causing fouling and obstruction of the same. Theexpansion also results in more or less rupturing of the electrodestructure at the surface, disintegrating and causing the same to crumbleand wear away unduly rapidly. Eventually, however, a stage is reached,after a considerable portion of the electrode has worn or crumbled away,when the remaining portion thereof acquires a hard, almost metalliccharacter, in which condition it is much more permanent and resistant tooxidation and is otherwise greatly improved in serviceability. It isdesirable to be able to produce the aforesaid improved structure of theelectrode artifically under controlled conditions before the same is putin service, thereby to avoid waste which results when the change iseffected during use in the electrolytic cell.

An improved method of treating electrodes of graphite and the like hasbeen described in our 'copending application, Serial No. 461,632, filedJune 16, 1930, now Patent No. 1,868,206. Briefly, this method consistsin first impregnating the electrodes with China-wood oil or other dryingoil and then subjecting the impregnated electrodes to an atmosphere ofchlorine gas for an extended time, which chlorinating step alone mayrequire a period of several weeks or even months.

By this treatment the oil-impregnated electrodes take upconsiderablechlorine, the oil thereof forming a chlorinated oil product which tendsto harden and more securely seal the pores of the electrodes. Thischlorinating operation is slow and takes place only at or near thesurface of the electrode owing to the practical impossibility of thechlorine penetrating deeply into a solid and relatively impervious body.The chlorination of the electrode is, therefore, more or lesssuperficial and not uniform and complete. The useful life of theseimpregnated and chlorinated electrodes, however, is known to beincreased in many instances as much as 50 per cent over that ofelectrodes impregnated but not so chlorinated. The method just referredto, although greatly superior to former methods of treating electrodes,yet has certain disadvantages, e. g. requiring a very large inventory ofboth electrodes and impregnating materials, and, furthermore, inyielding only an incompletelyv and non-uniformly chlorinated electrode.

We have now invented a new and improved chlorinated oil impregnatedcarbon or graphso itized electrode and a process for making same, suchelectrode being particularly adapted for use in the electrolysis ofalkali metal chloride solutions and embodying advantageous featuressuperior to those hitherto'employed. Our inven- 5 tion, then, consistsin the product and process hereinafter fully described and particularlypointed out in the claims, the following description setting forth indetail one mode of carrying out the invention, such disclosed modeillustrating, however, but one of various ways in which the principle ofthe invention may be used.

According to our invention, carbon or graphitized electrodes areimpregnated with a suitably prechlorinated drying oil. The oil, e. g.Chinawood oil, is first chlorinated directly or dissolved in a suitablediluent, preferably at a temperature of about 30 to 70 C., care beingexercised that the temperature during chlorination does not rise to apoint at which the oil materially thickens or becomes sticky. Thechlorination probably takes place largely by addition at the doublebonds of the unsaturated drying oil, but some substitution may alsooccur. The treat- 10$ ment is regulated by observing the decrease of theiodine number of the oil. For instance, chlorination of a mixture ofChina-wood oil and kerosene in about equal parts, may be continued untilthe iodine number has been reduced from an original value of about132436 to a value approximately 0 to 15. The practical re sultsobtained, however, are substantially in proportion to the degreeofchlorination. For different oils or mixtures thereof, differentdegrees of chlorination may be found to give the best results. As afinal test we may also determine the amount of chlorine contained in thefinal product.

The electrodes to be treated are first cleaned of foreign matter, suchas dirt, etc., and are introduced into a suitable impregnating vessel.

in which they are subjected to a vacuum. The vacuum connection to theautoclave is then closed and the chlorinated oil prepared as described.is drawn into the autoclave until the electrodes are nearly or entirelycovered. The autoclave and contents are then preferably sub jected topressure to facilitate the impregnation with the chlorinated oil. Whenimpregnation is complete the pressure is released and the vacuum appliedas before, after which the vacuum is released and the unabsorbed excessoil blown out by a stream of air or in other suitable manner, theelectrodes then being cooled and discharged. Some unabsorbed oil maycling to the electrodes, which upon exposure to-the air hardens and maybe removed by Scraping, buffing or by other mechanical operation. Theimpregnated electrodes may also be cleaned by washing with a solution ofNaOH of proper concentration or with other suitable solvent or cleaningcompound. I

By this impregnating treatment the pores of the electrodes are uniformlyfilled and closed with the chlorinated oil and a e thereby renderedimpervious to the penetra ion of the electreated in this way have beenfound to have a.

much longer life than those prepared according to methods heretofore inuse. The protective action of the chlorinated oil is distributedthroughout the body of the treated electrodes, instead of being limitedto a layer at the surface. Furthermore, bleeding, sweating or exuding ofoil due to chlorination in situ during use is practically entirely doneaway with.

The following example illustrates a preferred mode of carrying out ourinvention, but the invention is not limited thereto:-

gallons of a mixture consisting of approximately equal amounts of-China-wood oil and kerosene was introduced 'into a chlorinating vesseland heated to about 30 C. A current of chlorine gas, which hadpreviously been dried by passing through sulphuric (acid, was nowbubbled through the mixture, the temperature gradually rising to around50 C., at which point the flow of chlorine was curtailed so as toprevent a further temperature rise. A chlorine pressure sufficient toovercome the hydrostatic pressure of the oil was maintained-in thechlorinating 'vessel for about 48 hours. The iodine number of the oilmixture, which before chlorinating was 136, was reduced to 15,corresponding to an actual chlorine content of approximately 16 percent.

100 pounds graphitized carbon electrodes were charged into animpregnator vessel, which was then closed and evacuated to an absolutepressure of about 1 to 2 pounds per square inch for two hours. Thevacuum line to the impregnator vessel was then closed and the vesselconnected to a tank containing the chlorinated mixed oils from the firststep, and without releasing the vacuum, the chlorinated mixed oilswere'drawn into the impregnator vessel. The oil charged was thensubjected to a gauge pressure of about 100 pounds for approximately 4hours, when the pressure was released. Vacuum was again ap-- plied for 1hour and then released, excess oil drained off and the impregnator blownwithair and cooled. The electrodes thus treated were then removed fromthe impregnator and cleaned. A chlorinated oil content of 6 per cent 7from the electrodes and the resulting good condition of the diaphragmsin the cells entirely equipped with the electrodes treated by thepresent improved process at the termination of the preliminary runs wasparticularly noticeable. By use of our improved electrodes, the usualtime of the preliminary runs on the first diav phragm on the cell, i.e.about one month, was extended to three months or more.

In the foregoing we have described an improved process for treatingporous electrodes, particularly graphitized electrodes for use inelectrolytic cells for the manufacture of chlorine, sodium hydroxide,etc., by electrolysis of an aqueous solution of sodium chloride. By ourprocess we are able to reduce the complete time of treating to a fewhours as against a period of weeks or months or more by other processeswherein a less satisfactory result is had.

It is noted that electrodes of varying size, shape, or condition ofporosity, might require more or less time for impregnating with a givenchlorinated oil or mixture of oils prepared according to the first stepof our improved process. Numerous oils or mixtures of oils in additionto those previously named are applicable, 6. g. lin

nor to a moderately elevated temperature, e. g. as high as 200 0., tofacilitate the penetration of the pores by the oil, while at the sametime subjecting the charge in the impregnating vessel to pressure. Aswill be apparent to those skilled in the art, the degree of chlorinationof the China-wood oil or equivalent drying oil may be varied within widelimits, a partial chlorination giving results in proportion to theamount of chlorine absorbed. It is considered that, in the- Theinvention also comprises a new and improved electrode characterized as acarbon or graphitized electrode that has been practically uniformlyimpregnated throughout with a chlorinated drying oil. Our improvedelectrode is further characterized as being substantially impervious tothe penetration of aqueous solutions of alkali chlorides or the productsof electrolysis thereof under the ordinary conditions of carrying 7 outsuch electrolysis. The tendency to sP nginess, softening, dusting,pitting, or premature breaking off in the cell, such as ischaracteristic of untreated or imperfectly treated carbon or graphitizedelectrodes, is also substantially absent therein.

Other modes of applying the principle of our invention may be employedinstead of the one explained, change being made as regards thecomposition and process herein disclosed, provided the ingredients orsteps stated by any of the following claims or the equivalent of suchstated ingredients or steps be employed.

We therefore particularly point out and distinctly claim as ourinvention:-

1. In a process for treating carbon or graphitized electrodes, the stepswhich consist in chlorinating a drying oil and impregnating saidelectrodes with such chlorinated oil.

2. In a process for treating carbon or graphitized electrodes, the stepswhich consist in chlorinating China-wood oil and impregnating saidelectrodes with such chlorinated oil.

3. A new article of manufacture, a carbon or graphitized electrodesubstantially uniformly impregnated with a chlorinated drying oil.

4. A new article of manufacture, a carbon or graphitized electrodesubstantially uniformly impregnated with chlorinated China-wood oil.

5. In a process for treating carbon or graphitized electrodes, the stepswhich consist in chlorinating China-wood oil to an iodine number between0 and and impregnating the electrodes with: such chlorinated oil.

6. As a new article of manufacture, a carbon or graphitized electrodeimpregnated with chlorinated China-wood oil having an iodine numberbetween 0 and 100.

7. In a process for treating carbon or graphitized carbon electrodes,the steps which consist in chlorinating China-wood oil to an iodinenumber between 0 and 100, impregnating the electrodes with suchchlorinated oil, and then cleaning the surface of the impregnatedelectrodes.

8. In a process for treating carbon or graphitized carbon electrodes,the steps which consist in chlorinating China-wood oil to aniodinenumber between 0 and 100, impregnating the electrodes with suchchlorinated oil, and then cleaning the surface of the impregnatedelectrodes by washing the same with a sodium hydroxlde solution.

RALPH M. HUNTER. LOUIS E. WARD.

